To dream that you are sucking on something implies that you are lacking some inner spirituality. You are trying to avoid your obligations. You may be feeling vulnerable and unworthy. The dream may also be a play on the words 'sucking up' to someone. Perhaps you are trying to manipulate someone for your own benefit.
To dream that someone is sucking on you implies that you are exhausted and fatigued. You are not getting as much out of a relationship as you are giving.